Incorporated by reference herein is a computer program listing appendix submitted on compact disc herewith and containing ASCII copies the following files: PCMMAP.C, 23 KB, created Aug. 7, 1998; MATH.C, 4 KB, created Mar. 17, 1997; PCM.H, 2 KB, created Mar. 17, 1997; PCMCNV.C, 7 KB, created Mar. 14, 1997; and STANDARD.H, 7 KB, created Mar. 14, 1997.
1. Field of the Invention
This invention pertains generally to modem technology, more specifically to PCM modem technology, and more specifically, to a method for the discovery of the digital attenuation from a set of received PCM samples corresponding to a known set of transmit PCM samples.
2. Background of the Invention
This invention provides a method for the discovery of the digital attenuation from set of received pulse code modulation CPCM) samples corresponding to a known set of transmit PCM samples. The principal feature set used is the knowledge of which received PCM codes have become indistinguishable (i.e., are identical) as a result of the attenuation mapping. An alternative but equivalent feature set would be use of the absent received PCM codes.
The question of digital attenuation PAD mapping has been raised at recent meetings of the Telephone Industry of America (TIA) PCM Modem Ad-hoc Committee. Many participants have been hoping for a single industry standard mapping solution. We provide a solution, which will allow PCM modems to function properly throughout the entire telephone network including with private telephone equipment (PBX""s and key systems).
TIA 464A is the industry standard plan for private telephone equipment. Among other recommendations, this specification provides the recommended losses between T1 lines and other devices. If a PBX is designed according to the loss-plan specification as outlined in sections 4.8.4 and 4.8.5 of 464A , then it should not be necessary to attenuate the PCM on incoming T1 lines for on-premise (ONS) connections, since the recommended 3-dB loss is typically implemented in the analog circuitry of the ONS codec.
However, Inter-Tel""s experience with customers using T1 lines suggests that the recommended 3-dB of insertion loss is inadequate and that T1 attenuation pads are indeed necessary. While the standard xe2x80x943 and xe2x80x946dB pads are usually adequate, Inter-Tel has provided a wider-range of insertion loss as options for PBX customers. Inter-Tel has implemented digital gain control on its T1 line cards using two different methods:
ROM-based mapping
DSP-based algorithm
Inter-Tel first implemented attenuation pads via an EPROM circuit, which used the incoming PCM code as 8 bits of an address and additional address bits selecting the attenuation in 1 dB increments. The output of the EPROM was the attenuated digital PCM code. In this implementation, the attenuation range covered from 0 dB to xe2x80x9412 dB in 1 dB steps.
Experience with customers shows that 0 to xe2x80x946dB seems to be the most-often used range of T1 pad values. Taking into account the fixed 3-dB of loss included with our ONS card, the net insertion loss for to Digital CO connection is thenxe2x80x943dB toxe2x80x949dB, which mimics well the typical net losses experienced by customers on ONS to Analog CO connections.
All attenuation tables in the EPROM preserved the LSB of the PCM to preserve the state of the Robbed Bit Signaling (RBS), if present. However, the preservation of the LSB is unnecessary if no RBS information passes through the ROM-based gain control circuitry.